In the early of 1960s, Y-type zeolites used at the soonest were the rare earth-exchanged REY zeolites. REY zeolites have a silica/alumina ratio of less than 5, a rare earth content of not less than 17 wt % (based on RE2O3, and relative to the weight of the zeolites), a high acid center density, and a strong hydrogen transfer performance, a low olefin and naphthene content in gasoline, so as to reduce the octane number of gasoline. Thus tetraethyl lead was obliged to being added as an additive for increasing the octane number thereof. Nevertheless, Y-type zeolite is still the main active component in the catalyst for FCC (Fluid Catalytic Cracking) of heavy petroleum hydrocarbons.
In 1975, U.S.A abrogated leaded gasoline, and the cracking catalysts in which ultrastable Y (USY) zeolites replace REY zeolites as the active component appeared. USY is a high silica Y zeolite prepared by framework dealuminization of Y-type zeolites via hydrothermal treatment, wherein said zeolite generally has a silica/alumina ratio of from 5 to 10 and contains no or a small quantity of rare earth. Since USY has an increased framework silica/alumina ratio, a reduced acid center density, and a weakened hydrogen transfer performance, gasoline has an increased olefin content and an enhanced octane number. For example, U.S. Pat. No. 4,242,237 discloses a cracking catalyst for producing gasoline having a high octane number, wherein the active components comprise a USY zeolite having a rare earth in an amount of less than 1.15 wt % (based on RE2O3, and relative to the weight of zeolites) and small pore zeolites including erionite, mordenite, A zeolite, chabazite and offretite. U.S. Pat. No. 4,259,212 discloses a cracking catalyst containing USY zeolites which comprise rare earth in an amount of less than 1.15 wt % (based on RE2O3, and relative to the weight of zeolites) and a unit cell constant less than 24.41 Å. USY zeolites used in said two patent documents both comprise a small quantity of rare earth. In the early 80s, ZSM-5 type zeolites began to be used in FCC catalysts for increasing the octane number of gasoline. U.S. Pat. No. 4,309,280 discloses that 0.01-1 wt % of HZSM-5 zeolites relative to the weight of the catalyst may be directly added into the FCC apparatus. U.S. Pat. No. 3,758,083 discloses a catalyst containing as active components ZSM-5 zeolites and large pore zeolites (such as X-type and Y-type) in a ratio of 1:30 to 3:1, which is used to increase the octane number of the gasoline product and enhance the C3=+C4= yield at the same time. The function of ZSM-5 during FCC is actually to crack straight chain hydrocarbons having a low octane number in the gasoline fraction into low carbon olefins, to aromatize a part of low carbon olefins, so as to increase the octane number of gasoline. Thus the application of ZSM-5 will unavoidably increase the olefin and aromatic hydrocarbon content in the gasoline. There are lots of patent documents regarding high silica Y zeolites as the activity component of the catalyst. For example, U.S. Pat. No. 4,880,787 discloses a zeolite catalyst containing USY having a silica/alumina of 5-100 and a constraint index of 1-12, wherein the support comprises aluminium and 0.01-10 wt % of rare earth elements relative to the weight of the catalyst. Such catalyst is primarily used for increasing FCC gasoline and distillate oil yields and reducing coke and dry gas yields during the FCC process.
The catalyst prepared from REY zeolites or USY zeolites having a broad silica/alumina ratio cannot magnificently meet the requirements on the selectivity of the FCC target product. When added into the commercial catalytic cracking unit, said REY-type zeolite catalyst or USY-type zeolite catalyst has an initial activity of higher than 85, wherein REY-type zeolite catalyst has an initial activity of higher than 90. Under the high strength hydrothermal treatment, the catalyst activity of said two kinds of zeolite catalysts gradually decreases. The catalyst activity of REY-type zeolite catalyst decreases straightly, while the initial activity of USY-type zeolite catalyst at first decreases quickly, and then slowly as the aging time increases (see Fluid Catalytic Cracking Handbook: Design, Operation, and Troubleshooting of FCC Facilities, Reza Sadeghbeigi, 2nd edition, P 92, FIGS. 3-5).
Along with the increase of the crude oil output, the quality of crude oil becomes worse mainly in the following aspects: crude oil density becomes greater; the viscosity thereof increases; the heavy metal content, the sulfur content, the nitrogen content, the resin and asphaltene content, and the acid number becomes higher. Currently, price difference between inferior crude oil and high-quality crude oil becomes great along with the shortage of petroleum resources. Thus much attention is paid to the method for exploiting and processing inferior crude oils having a low cost, i.e. increasing the yield of light oils from inferior crude oils as much as possible, which brings about great challenge to the conventional processing technology for crude oil. In order to meet the increasing need for light olefin chemicals and motor gasoline, PCT/CN2009/000272 discloses a process for producing light fuel oil and propylene from inferior feedstocks. Inferior feedstocks are fed into the first and second reaction zone of the catalytic converting reactor in turn, are in contact with the catalytic converting catalyst to carry out the first and second reactions. After the gas-solid separation of the reaction product and the spent catalyst, the spent catalyst is stripped, coke-burned and then recycled into the reactor. After the separation of the reaction product, propylene, gasoline, fluid catalytic cracking gas oil (FGO) and other products are obtained, wherein said fluid catalytic cracking gas oil is fed into the aromatic extraction unit to obtain the extracted oil and raffinate oil by separation. Said raffinate oil is recycled to the first reaction zone of the catalytic converting reactor or/and other catalytic converting devices for further reaction to obtain the target products, i.e. propylene and gasoline. In said process, FGO obtained after moderate catalytic conversion of inferior feedstocks is separated by using an aromatic extraction unit. Bicyclic aromatic hydrocarbons are enriched in the extracted oil, so that the extracted oil is an excellent chemical. Alkanes and cyclanes are enriched in the raffinate oil, so that raffinate oil is very suitable for catalytic conversion, so as to achieve the high efficient utilization of petroleum resources. Said process can greatly reduce the dry gas and coke yield, and the catalyst used in said process is mainly based on the selectivity of the catalyst for the target product.
Due to continuous abrasion, the catalyst in the commercial catalytic cracking unit drains away during the operation. In addition, a part of the equilibrium catalyst is usually unloaded in order to maintain the equilibrium catalyst activity as required by the reaction. Meanwhile, it is forced to supplement fresh catalyst (currently, the activity of a generally fresh catalyst is higher than 85, and the selectivity of the dry gas and coke is extremely worse). Thus there is the reasonable makeup rate of fresh catalyst to the inventory of the system equilibrium catalyst. It can be seen that the equilibrium catalyst is the result of the combined effect of the continuous addition of fresh catalyst and the continuous loss (including artificial unloading) of the system equilibrium catalyst. Currently, fresh catalyst is usually supplemented into the catalytic cracking unit by the following method. That is to say, fresh catalyst from the fresh catalyst storage tank is fed into the hand feeding instrument or automatic feeding instrument, weighted, and discharged after air-venting and fluidization, and then the catalyst is delivered to the regenerator of the catalytic cracking unit. As for how to achieve the autoweighting of the catalyst, and how to reduce the equipment failure during the automatic feeding process, there are many patents documents. For example, CN1210029A discloses a small-size automatic feeding system for the catalytic cracking catalyst.
Hydrothermal inactivation of a catalyst is a slow process having an average lifetime of 30-100 days. During the inactivation process, the activity of the fresh catalyst, the metal content of the feedstock oil and other properties thereof, the operating conditions of the FCCU (Fluid Catalytic Cracking Unit), the wastage and dump rate of the catalyst cannot remain constant. Meanwhile, fresh catalyst in the single particle form loses the physical and chemical properties thereof at the moment of the entry into the complete-mixing-flowing regenerator. Due to these problems, it is difficult to accurately predict the age distribution and activity distribution of the catalyst in the commercial catalytic cracking unit. By directly taking the equilibrium catalyst sample in the commercial catalytic cracking unit for measuring the activity of the equilibrium catalyst or other properties, or calculating the activity or other properties of the equilibrium catalyst on the basis of the simplified mathematical model, the resultant activity or other properties of the equilibrium catalyst are merely the average values of the average activity or other properties. These values are the key parameters for instructing the production operation of the FCCU and optimizing the product distribution and property. However, there occurs a serious problem at the same time, i.e. overlooking the difference in the effects of each and every catalyst particle in the commercial catalytic cracking unit on the product distribution and property. CN1382528A discloses a cyclic polluting and aging process for catalyst. After treatment by said process, the physicochemical properties of a fresh catalyst all are close to the industrial equilibrium catalyst. Said process is mainly designed for the difference between the catalyst processed in laboratories and the industrial equilibrium catalyst, but it is unable to improve the activity difference between the industrial equilibrium catalysts. CN1602999A discloses a method of exterior pre-treatment for hydrogenation catalysts, comprising the steps of ex-situ prevulcanization of gas phase of hydrogenation catalyst in oxidation state, passivating the catalyst in sulfidization state by using the oxygen containing passivation gas. Said method can notably increase the activity and stability of the catalyst. However, said method is merely suitable for the treatment of hydrogenation catalyst, rather than the catalytic cracking catalyst.